how far apart should i put new joists under my floor?

I always do 400mm centres, but depending on joist size you could go up to 600mm centres. It's always better to do closer centres as it reduces the bounce in the floor. Also if doing new chipboard flooring it's always better to land the join on a joist and stagger joins on the next run. If it's a hole new ground floor joist you can buy a damp proof membrane which would stop rising damp

The joists are best set at 400 centres, I’ve found that using 22mm chipboard decking helps also helps greatly with taking any bounce away, only if you’re able to do so with the thicker boards and you can purchase moisture resistant boards, there not water proof!! their water resistant a big difference 👍

As with guys above I agree always set joist at 400 centres. I would fit 22mm wayrock moisture resistant boards staggering the joints. As for the damp if it is only coming through the floor, fitting a 2000 gaged dpm correctly should eradicate the damp issue